The components of high-temperature mechanical systems, such as, for example, gas-turbine engines, must operate in severe environments. For example, the high-pressure turbine blades and vanes exposed to hot gases in commercial aeronautical engines typically experience metal surface temperatures of about 1000° C., with short-term peaks as high as 1100° C.
Typical components of high-temperature mechanical systems include a Ni- or Co-based superalloy substrate. The substrate can be coated with a thermal barrier coating (TBC) to reduce substrate surface temperatures. The thermal barrier coating may include a thermally insulative ceramic topcoat, and may be bonded to the substrate by an underlying metallic bond coat.
The component may be exposed to widely different temperatures, e.g., up to 1100° C. during operation and on the order of 25° C. when operation is ceased. These widely different temperatures may cause significant stress at the interface of the bond coat and thermally insulative ceramic topcoat, which eventually may lead to spallation of the TBC from the substrate. The stress may be due to, for example, oxidation of the bond coat and/or the bond coat and thermally insulative ceramic topcoat having different coefficients of thermal expansion. In order to improve maintenance of the high-temperature mechanical systems, it may be desirable to estimate when the TBC is expected to fail and should be repaired or the component be replaced.